This invention relates to charged particle beam recording on a film as disclosed in copending U.S. application Ser. No. 023,548, now U.S. Pat. No. 4,300,147, and more specifically to an improved system and film gate for more accurately producing an image on an electron sensitive film with an electron beam. Such systems find particular utility in the graphic recording industry and provide accurate and high resolution imaging. Thus, electron beam recording is especially useful for, producing computer output microfilms (COM), mass data storage, wide bandwidth signal recording, satellite photography, automated cartography as well as computer micrographic and seismic recording.
Essential elements of an electron beam recorder (EBR) include an electron gun, an electromagnetic focusing and deflection system, an electron sensitive film and a film gate. In operation, the electron beam traces or plots an image in the recording area on the emulsion side of a stationary electron sensitive film accurately positioned in the film gate. Electron beam recording is conducted in a vacuum atmosphere because air can scatter or attenuate the electron beam. In a particular embodiment, a three stage vacuum system is used in an electron beam recorder system with the volume with highest vacuum containing the electron gun, a lower vacuum volume containing a beam path to the emulsion side of the recording film and the lowest vacuum in the volume containing the film dispenser and transport mechanisms. This three stage vacuum system provides for rapid access to the film for the easy changing or replacement thereof. However, it has been discovered that at times a small distortion occurs in the image on the film and in some applications of the inventions these distortions limit its value. It is believed that the differential pressure exerted on the film by the three stage vacuum can adversely affect the shape of the film because it creates variable and unpredictable buckling of the film in the film gate. Moreover, from time to time, the film may be further distorted by air entrapped behind the film that may originate from the outgassing of water vapor or other atmospheric constituents contained in the film or by air which is transported by the film from the lower vacuum volume into the film gate as the film is advanced to provide fresh film for exposure.
It is recognized that instabilities and uncertainties in the position of the recording film relative to the beam are a primary limitation on the geometric accuracy with which an image can be recorded on the film by high performance EBRs.
As disclosed in copending U.S. patent application Ser. No. 023,548, now U.S. Pat. No. 4,300,147, the disclosure of which is incorporated by reference herein, an EBR film gate can be made having a film receiving surface shaped to a predetermined configuration, so as to precisely position a recording film along a predetermined continuous curvature. The continuously curved film gate may also include a means for continuously and uniformly tensioning the recording film to conform the film to the predetermined continuous curvature of the film gate. Thus the distortions which have heretofor caused instabilities and uncertainties in the position of the recording film have been eliminated and the film is controlled so that these distrotions conform to a known geometric form and are thereby made predictable. In operation, the electron beam is electronically controlled to compensate for these predictable and known geometric distortions to create an essentially geometrically distortionless image on the recording film.
While the geometrically distorting film gates in the aforementioned applications have improved substantially the geometric accuracy of the EBR recording, that film gate has not completely eliminated those film distortions which are not correctable by a film gate which only geometrically distorts the film. More particularly, it has been found that the charged particle recording film does not always lie contiguous with or flat against the film receiving surface of the film gate platen and is sometimes distorted because it is spaced therefrom by air or gases trapped between the film and the film receiving platen which form bubble like protrusions. The bubble is due to the fact that the electron receiving face of the film is exposed to a higher level vacuum than the rear face of the film which is in contact with the lower level vacuum present in the volume occupied by the film transport mechanism. Even though trapped air or gases which can cause the bubble like effect represents a substantially less significant source of film distortion than for instance film buckling or film outgassing, it is desireable to reduce or eliminate this bubble like effect.